Ladle metallurgy is perhaps the most widely used practice to enhance metal quality and productivity employed in the steel industry today. The term "ladle metallurgy" when used herein will mean post-melting processes which control post-melting temperatures and/or lower or control gaseous and metallic constituent contents of the steel. Among the processes currently known today are the following.
The vacuum arc degassing process, or the VAD process as it is often referred to, includes the subjection of a melt of steel to vacuum, a purging agent, such as an inert gas, and an alternating current heating arc struck and maintained between non-consumable electrode means and the molten steel. The process is described in several patents assigned to the assignor of this application, including U.S. Pat. Nos. 3,501,289, 3,501,290 and 3,635,696.
The so-called ladle furnace is also in use. This system includes the heating of molten steel at one station, usually by conventional electric arcs operating under conventional conditions including ambient atmosphere, and stirring (usually by an induction stirrer, but, also, sometimes assisted by a gas purge), plus a vacuum at a second station. This system has been offered by the Swedish firm ASEA.
Another furnace type system is the so-called Daido ladle furnace which involves, essentially, heating, usually with electric arcs, in an open or shrouded ladle using purging gas for stirring and a special slag to protect the bath. This process is believed to be exemplified by U.S. Pat. Nos. 4,371,392, 4,308,415 and 4,272,287.
And many steelmakers are employing do-it-yourself furnace heating systems by utilizing surplus parts from displaced arc furnaces, usually with the arc furnace shell replaced by a ladle.
All of these exemplary systems have certain common characteristics to a greater or lesser extent as follows.
All systems create fumes which must be captured, particularly during operation of the heating arc. Specifically, some fumes are quite dangerous, including, for example, carbon monoxide. The VAD process removes said fumes in a highly efficient manner since they are exhausted through the vacuum system, but the other systems appear to be somewhat less efficient in this respect.
The arcing in all of said systems may create a noise problem.
Those systems which require a vacuum at some stage in the process have the substantial additional capital costs connected with vacuum equipment and, of course, added maintenance costs associated with such equipment.
Those systems which utilize an unsealed container provide, unfortunately, ideal conditions for moisture pick-up by the steel, which primarily manifests itself as an increase in the hydrogen content, though an increase in oxygen content is also possible. Indeed, experience has shown that in a ladle which is partially enclosed (which may be the case when a normally vacuum tight tank fails to seal properly), the arcing causes a series of small explosions, or pulses, at a rate of on the order of about one per second and an amplitude of about plus or minus one-quarter inch water when observed on sensitive magnahelic pressure gauges. The explosion will blow gases out of the enclosure but likewise for each explosion there will be an implosion at less than atmospheric pressure which results in moist ambient air being sucked into the enclosure. The water vapor in the air yields its hydrogen to the steel. The amount of hydrogen pickup from the moist air need not be large to render the product unacceptable. For example, in a 65 ton heat of low alloy steel, the addition of a six ounce glass of water which will thereby raise the hydrogen content from the flake-free level of 2.0 ppm to the flake-sensitive level of 2.5 ppm can result in rejection of the final product. It will be understood that the pressure of the arcs bares the molten metal under the arc, thereby providing ideal conditions for the pick-up of hydrogen from the pulse of moist air which follows each explosion and consequent break in the integrity of the sealed enclosure.
It should also be noted that the explosion/implosion phenomenon in a well enclosed ladle, or even simply thermal drafts from a shrouded ladle, preclude the possibility of graphite or carbon from the electrodes burning out the oxygen and providing a very low oxygen environment above the bath.
And further, it should be noted that all such techiques are temperature sensitive. Even with ladle preheat, the initial heat sink of the ladle can cause stratified temperature levels which are deleterious to casting and solification rates, especially when the unit is used in conjunction with a continuous casting machine. Stirring ladles with an inert gas for temperature uniformity helps reduce the temperature stratification problem, but the stirring hastens the temperature drop in the ladle. Costly overheating in the melting furnace takes time and accelerates furnace refractory wear and renders processing so uneconomical that it is now avoided if at all possible.